Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 16
RAID level 0/1 sector interleaved groups of mirrored disks; this is
sometimes called RAID level 1/0 or RAID level 10
Since there is no data protection with RAID level 0, the only benefit is the potential for
increased performance due to the data being spread across multiple disks. HP has
implemented a special case of RAID level 0 called Independent Mode, in which the
interleaving group size is one, effectively resulting in each disk being treated as if it
were a non-RAID disk. Independent mode can sometimes increase performance over
RAID level 0 in cases of small random I/Os.
Since the sector size on most disks is 512 bytes, the minimum I/O size is 1 Kbyte for 2
disks in a group or 2 Kbyte for 4 disks in a group for RAID levels 0 and 3, not counting
the parity disk with RAID level 3. RAID 0 is usually implemented with groups of two
disks. RAID level 3 has been implemented by HP with groups of 3 or 5 disks, one of
which is the check disk.
HP-UX imposes a 1 Kbyte minimum I/O size requirement for disk drives. So, even with
RAID level 5, the minimum I/O size is 1 KB. However, depending on the particular
implementation, an I/O from 1 KB to 128 KB can involve a single disk spindle.
Modern RAID implementations include levels 0, 1, 0/1, 3 and 5 only. Disk arrays
support one or more RAID levels and may allow several RAID levels to operate
concurrently, such as with the High Availability Disk Arrays. RAID disk arrays are built
so that the disk mechanism is encased, and can be removed and inserted easily and
correctly without disconnecting the SCSI bus or affecting I/Os in progress.
HP-FL Disk Arrays
HP-FL disk arrays, henceforth called FLDA for brevity, are available from HP as four
different products.
C2258HA (three 2 GB disk mechanisms with data protection)
C2258B (two 2 GB disk mechanisms without data protection)
C2259HA (five 2 GB disk mechanisms with data protection)
C2259B (four 2 GB disk mechanisms without data protection)
Only two of these products, with the suffix "HA", provide data protection. The others
must be used with MirrorDisk/UX for data protection.
Internally, the FLDAs use 5-1/4 ", 5400 RPM SCSI disks and SCSI controllers in 1 and
2 GB versions. However, the external connection is HP-FL whose advantage is
distance from the computer system up to 500 meters. When two FLDAs are daisy-